The input quill bearing housing was not positioned in accordance with the procedures described by the manufacturer; therefore, the flow of oil was obstructed, causing the catastrophic failure of the input quill bearings. Independent inspection did not detect the incorrect reassembly of the main transmission gearbox.Findings as to Causes and Contributing Factors The input quill bearing housing was not positioned in accordance with the procedures described by the manufacturer; therefore, the flow of oil was obstructed, causing the catastrophic failure of the input quill bearings. Independent inspection did not detect the incorrect reassembly of the main transmission gearbox. There are no mechanical means to prevent an installation error when installing the input quill bearing housing. The force required to shear the main rotor shaft is higher than the force required to shear the six rotor head attachment bolts. As a result, the rotor could separate from the shaft in the event of a sudden stoppage of the transmission, which constitutes a hazard for helicopter occupants and people on the ground.Other Findings There are no mechanical means to prevent an installation error when installing the input quill bearing housing. The force required to shear the main rotor shaft is higher than the force required to shear the six rotor head attachment bolts. As a result, the rotor could separate from the shaft in the event of a sudden stoppage of the transmission, which constitutes a hazard for helicopter occupants and people on the ground. At the completion of the main transmission overhauls or at sudden stoppage inspections or in any other situations in which the retainer has to be removed, AOG Heliservices Inc. will paint a red witness line on the retainer and on the transmission housing to assure alignment of oil ports. Also, they will run the transmission for fifteen minutes to check that there is oil flow in the transmission and to check for oil leaks at the seal and split line. These changes will be put into their worksheets.Safety Action Taken At the completion of the main transmission overhauls or at sudden stoppage inspections or in any other situations in which the retainer has to be removed, AOG Heliservices Inc. will paint a red witness line on the retainer and on the transmission housing to assure alignment of oil ports. Also, they will run the transmission for fifteen minutes to check that there is oil flow in the transmission and to check for oil leaks at the seal and split line. These changes will be put into their worksheets.